For the controlled release of drugs such as aripiprazole, microspheres composed of a drug and a base polymer are used. Various conventionally known microspheres are of the matrix type in which a drug is substantially uniformly distributed in a base matrix such as a polymer.
For example, base/drug matrix-type microparticles are obtained by dissolving a base and a drug together in a solvent, drying the solution as such, followed by compression and fragmentation (Patent Document 1). However, when such a matrix-type microsphere has a high drug content, e.g., a weight ratio of drug to base of 1 or more (i.e., a drug content of 50% by weight or more), the drug constitutes a large part of the microsphere. Consequently, a large amount of the drug also exists on the surface of the microsphere. It is generally considered that a large amount of the drug present on the surface of the microsphere disables control of release by the base.
According to another known process, a drug and a base are dissolved in an organic solvent such as dichloromethane, an O/W emulsion is prepared in an aqueous system, and dichloromethane is vaporized (Patent Document 2).    [Patent Document 1] US 2004/0247870 A1    [Patent Document 2] WO 94/10982